1. Field of the Invention
This invention relates to liquid crystal light shutters and methods for producing such devices. More specifically, monomers and liquid crystals are mixed; polymerization causes microdroplets of liquid crystals to form by phase separation. Wide viewing angle normal or reverse mode displays are provided by using monomers having differing reactivities.
2. Description of Related Art
Liquid crystal displays (LCDs) have grown rapidly in importance in recent years. The LCD is now second only to the cathode ray tube in the market for displays. Many other applications, such as switchable windows in buildings and automobiles, large advertising displays and other uses are being considered. The property of liquid crystals that makes the material useful in displays is its birefringence. This property can be used to form devices having the ability to transmit or to scatter or absorb light, dependent on whether an electric field is applied to the material.
One of the new developments in LCDs in recent years has been polymer dispersed liquid crystal (PDLC) displays. In this type LCD, liquid crystal material is contained in microdroplets embedded in a solid polymer matrix. Such displays are formed by phase separation of low-molecular weight liquid crystals from a prepolymer or polymer solution to form microdroplets of liquid crystals. A survey of these materials is provided in Liquid-Crystalline Polymers, ACS Symposium Series 435, Chap. 32, pp. 475-495, 1990. Another approach to obtaining dispersed microdroplets in a polymer matrix is the method of encapsulating or emulsifying the liquid crystals and suspending the liquid crystals in a film which is polymerized.
Birefringence results from a material having a different index of refraction in different directions. The extraordinary index of refraction (n.sub.e) of a liquid crystal molecule is defined as that measured along the long axis of the molecule, and the ordinary index of refraction (n.sub.o) is measured in a plane perpendicular to the long axis. The dielectric anisotropy of liquid crystals is defined as .DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..perp., where .epsilon..sub..parallel. and .epsilon..perp. are parallel and perpendicular dielectric constants, respectively. Liquid crystals having a positive dielectric anisotropy (.DELTA..beta.&gt;0) are called positive-type liquid crystals, or positive liquid crystals, and liquid crystals having a negative dielectric anisotropy (.DELTA..beta.&lt;0) are called negative-type liquid crystals, or negative liquid crystals. The positive liquid crystals orient in the direction of an electric field, whereas the negative liquid crystals orient perpendicular to an electric field. These electro-optical properties of liquid crystals have been widely used in various applications.
A normal mode display containing liquid crystals is non-transparent (scattering or absorbing) in the absence of an electric field and is transparent in the presence of an applied electric field. Normal mode liquid crystal displays can be fabricated by the encapsulating or emulsion process, as described, for example, in U.S. Pat. Nos. 4,435,047, 4,605,284 and 4,707,080. This process includes mixing positive liquid crystals and encapsulating material, in which the liquid crystals are insoluble, and permitting formation of discrete capsules containing the liquid crystals. The emulsion is cast on a substrate, which is precoated with a transparent electrode, such as an indium tin oxide coating, to form an encapsulated liquid crystal device.
Normal mode polymer dispersed liquid crystal (PDLC) displays can also be fabricated by a phase separation process. This process, described in U.S. Pat. Nos. 4,685,771 and 4,688,900, includes dissolving positive liquid crystals in an uncured resin and then sandwiching the mixture between two substrates which are precoated with transparent electrodes. The resin is then cured so that microdroplets of liquid crystals are formed and uniformly dispersed in the cured resin to form a polymer dispersed liquid crystal device. When an AC voltage is applied between the two transparent electrodes, the positive liquid crystals in microdroplets are oriented and the display is transparent if the refractive index of the polymer matrix (n.sub.p) is made to equal the ordinary index of liquid crystals (n.sub.o). The display scatters light in the absence of the electric field, because the directors (vector in the direction of the long axis of the molecules) of the liquid crystals are random and the refractive index of the polymer cannot match the index of the liquid crystals. Nematic liquid crystals having a positive dielectric anisotropy (.DELTA..beta.&gt;0), large .DELTA.n, which may contain a dichroic dye mixture, can be used to form a scattering and absorbing mode.
A reverse mode display is transparent in the absence of an electric field and is non-transparent (scattering or absorbing) in the presence of an applied electric field. A recently-developed reverse mode microdroplet liquid crystal light shutter display is disclosed in U.S. Pat. No. 5,056,898. This display is said to have a reverse effect in modulating optical states. In order to achieve a reverse effect, the emulsion process was utilized and two major steps were required. First, nematic negative liquid crystals and a dopant, such as silane, were dispersed in a liquid polymer solution which had a high surface free energy, to form microdroplets of liquid crystals in the polymer matrix after solvent removal. Second, after the droplets were formed, the surface free energy of the polymer which encased the liquid crystals and dopant was modified by a reaction of the dopant with functional groups in the inner surface of polymer wall. The newly-formed inner surface layer has a low surface energy and aligns the nematic liquid crystals perpendicular to this layer. The structure of the display is multi-layers of flat microcapsules with a homogeneous material as the capsule skin.
If nematic negative liquid crystals are aligned perpendicular to the inner surface layer, but not to the substrate surface of a display, liquid crystals in microdroplets are not entirely perpendicular to the substrate. Ends of the elongated droplets contain liquid crystals that are not perpendicular to the substrate. The central part of liquid crystals in the droplets is clear if the refractive indexes of the polymer and inner surface layer match the ordinary refractive index of the liquid crystals (n.sub.o) at off-state. However, liquid crystals near the ends of the microdroplet are strongly bent because they are perpendicular to the skin of the inner layer. They are, therefore, tilted to the substrate surface, and the refractive index of the liquid crystals cannot match With the refractive indexes of the polymer matrix and inner layer. Therefore, parts of the liquid crystal droplets scatter light and produce haze.
Some polymer dispersed liquid crystal displays are formed by a solvent evaporation method. A disadvantage of the method employing solvent evaporation to form a display is the long time to evaporate solvents from coatings. If a dopant is used, the reaction between the polymer skin and the dopant may require a long time. Also, this method requires a protective polymer layer to prevent evaporation of liquid crystals in microdroplets during heating under vacuum for evaporation of solvent. Therefore, a higher voltage is normally required to drive the display, which is not desirable.
There is a need for a method of forming displays which does not require solvent evaporation, and which allows formation of a reverse mode display by simultaneous formation of liquid crystal microdroplets which are aligned perpendicular to a substrate and in a polymer matrix. There is also a need for displays, both normal and reverse mode, which have a minimum amount of haze and a wide viewing angle.